How The Combustion of Wood Proceeds
Firewood, one of the fuels for which the stove of this invention is designed, is composed of (a) solid flammable materials, and (b) other materials that are driven off from the fuel as flammable gases when the fuel is heated and burned during combustion. The stove of this invention is designed for efficient combustion of both types of fuel--flammable solids, and flammable gases driven off from the solid fuel during heating and combustion. Moreover, it is designed for fully automatic movement from a rapid burning mode of operation to a medium burning mode or a banked mode, as selected by the user, and back again to the rapid burning mode, in response to temperature responsive control means.
The constituents of wood include cellulosic materials, lignin, resin, oils, various extraneous materials such as tannin, and ash material. The process of combustion of these various constituents proceeds through four different temperature ranges. When the wood is first ignited, water is driven off as water vapor at about 200.degree. to about 250.degree. F. Pyrolysis of the wood--the chemical decomposition of the wood by the action of heat--proceeds in the temperature range of about 500.degree. to about 750.degree. F., producing charcoal, wood gas and wood oil vapors. The wood gas (containing carbon monoxide, methane, hydrogen, etc.) and the wood oil vapors ignite at about 1100.degree. F. and above. Finally, at about 1200.degree. to about 1800.degree. F., the charcoal resulting from the pyrolysis of the wood combines with the oxygen of the air to form carbon dioxide and, if the oxidation process is not completed, carbon monoxide.
The steps of pyrolysis of the wood and the oxidation of the resulting charcoal are commonly referred to as constituting "primary combustion," and the burning of the flammable gases resulting from the heating and combustion of the wood, including wood gas and wood oil vapors, is commonly referred to as "secondary combustion." It will be seen that in fact these two types of combustion may take place either consecutively or simultaneously, and thus may be either separate or mixed phenomena.
Despite this fact, workers in this field sometimes speak as if the two phenomena are always wholly separate and distinct. The reason for this may be that air introduced for the main--though not the sole purpose--of supporting primary combustion is conveniently referred to as "primary air," while air introduced through a separate inlet for the main purpose--though sometimes not the sole purpose--of supporting secondary combustion is frequently for convenience referred to as "secondary air."
This terminology distinguishing between primary and secondary air is used (for reasons that will be apparent from the context) in certain places in this specification, although reference is sometimes made to "so-called" primary air and "so-called" secondary air in order to stress the fact that what is commonly called "primary air" practically always contributes to a limited extent to secondary combustion, and what is commonly called "secondary air" can--depending upon the level of its introduction into the fire chamber--contribute to a limited extent to primary combustion. In other contexts in this specification, the description of a particular source of air is given in terms of the level at which the air is introduced into the stove, rather than the type of combustion for which the air is primarily introduced.
Coal, whether hard or soft, is another fuel for which certain stoves constructed according to this invention are designed to be used. Both hard coal and soft coal, like firewood, are composed of (a) solid flammable materials, and (b) other materials that are driven off from the fuel as flammable gases when the fuel is heated and burned during combustion. Depending on the particular type of coal being used, varying amounts of carbon, volatile material (under which heating is driven off in the form of gas, water vapor, light oil and tar), and impurities such as sulfur, phosphorous and incombustible rock materials (which upon burning produce undesirable gases or ash) are included in the coal.
Because of its composition as just described, the burning of coal, as in the case of firewood, involves two steps which are commonly referred to as "primary combustion" and "secondary combustion." Hence the operation of the stove of this invention is similar whether it is designed exclusively for the burning of firewood or is designed for the burning of coal or other similar fuels as well.